The digital audio interface in a GSM system (Global Mobile System) is a specified interface between a mobile station (MS) and a GSM system simulator (SS).
This interface is used in the conformance testing of a GSM device, especially in performing the bit exact test of the speech coder/decoder and in testing the sending/receiving loudness ratio (SLR/RLR). FIG. 1 represents the MS test arrangement of a mobile station wherein the essential parts with respect to the invention are marked: in the receive direction speech coder 101, digital interface 106, digital-analog converter 105 and speaker 108, and in the transmit direction microphone 102, analog-digital converter 103, as well as digital interface 107 and speech encoder 104. According to the specification the interfaces transfer 13 bits wide linear PCM data in serial mode at 104 kbits/s. It is also possible to interface, via digital audio interface 106/107, to the system simulator SS (not shown) to connect the speech data in both the transmit and receive directions. By means of the digital audio interface it is possible to observe and control the data stream in between the D/A and A/D conversion and the coding/decoding of the digital speech signal.
In FIG. 1 the internal test loops, i.e. test paths, of the MS device required by the specification are represented by reference numbers 1 to 4. Path 1 transmits decoded speech from speech decoder 101 of the MS device to simulator SS; this path is used for down link speech transcoding testing. Path 2 transmits a signal converted by A/D converter 103 and originating from acoustic input 102 to the simulator; this path is used for the acoustic devices test. Path 3 transmits a signal from simulator SS to speech encoder 104 of the MS device; this is used for up link speech transcoding testing. Finally, path 4 transmits a signal from the simulator to A/D converter 105 and to the acoustic speech output device (speaker); this path is used for the acoustic devices test.
In digital interface 106/107 (DAI), according to the specification represented in FIG. 1, the linking of the system simulator is a very clear function because the specification is based on the assumption that the D/A and A/D converters on one hand, and the coders/decoders on the other hand, are separate physical devices with interface DAI in between.
However, the applicant has, for different reasons, reached a more cost-effective solution in realizing the mobile station MS, wherein D/A and A/D converters and coders/decoders are implemented in one physical unit. This type of unit comprises a DSP processor for the speech transcoding function and an analog codec for the D/A and A/D conversion of the speech signal, as well as codec data register CDX, through which the system simulator SS has to interface to the MS device in order to meet the DAI specifications.
However, this creates problems. One alternative is that the DSP processor reads and writes into the data register CDX and subsequently interfaces to the system simulator via a serial SIO interface (not shown). However, this means that a specific program would have to be installed in the DSP processor that takes care of transferring the speech data from the SIO port to the codec data register and vice versa. This would result in a disadvantageous situation, where the MS would not be working in normal mode (i.e. would not be working with the normal mode speech coding program) during testing. Furthermore, the use of the serial port SIO for testing purposes limits the potential future use of the port for other purposes.